1. Field of the Invention
The present invention relates to a surge protection device, and particularly to a surge protection device to avoid a surge voltage into a load.
2. Description of the Related Art
With reference to FIG. 3, a conventional surge protection device 30 comprises a power port Vin, a ground port GND, a rectifying diode D, a Zener diode ZD, a resistor R, a transistor Q, a positive port 31, a negative port 32, and a surge protection unit 33.
The rectifying diode D has an anode electrically connected to the power port Vin, and has a cathode electrically connected to the positive port 31. The surge protection unit 33 has a positive terminal electrically connected to the cathode of the rectifying diode D, and has a negative terminal electrically connected to the ground port GND. The Zener diode ZD has a cathode electrically connected to the positive terminal of the surge protection unit 33 and the cathode of the rectifying diode D, and has an anode electrically connected to the ground port GND through the resistor R.
When a surge voltage between the positive terminal and the negative terminal is greater than a threshold voltage of the surge protection unit 33, the surge protection unit 33 is conducting to transform electric power of the surge voltage to heat power for releasing the surge voltage.
The transistor Q has a gate electrically connected to the anode of the Zener diode ZD, has a source electrically connected to the ground port GND, and has a drain electrically connected to the negative port 32.
The positive port 31 and the negative port 32 are electrically connected to a load such as an LED device 40.
A normal voltage for the conventional surge protection device 30 is greater than a breakdown voltage of the Zener diode ZD and is smaller than the threshold voltage of the surge protection unit 33. When the power port Vin receives the normal voltage, the Zener diode ZD is conducting and the surge protection unit 33 is not conducting. After the Zener diode ZD is conducting, a current flows through the resistor R to generate a voltage across the resistor R. Since the voltage across the resistor can make the transistor Q conducting, the normal voltage can be provided to the LED device 40 through the transistor Q. The LED device 40 receives the normal voltage to emit light normally.
With reference to FIG. 4, the surge voltage is greater than the breakdown voltage of the Zener diode ZD and greater than the threshold voltage of the surge protection unit 33. When the surge voltage occurs, the surge protection unit 33 is conducting to release the surge voltage to protect the LED device 40 from being damaged.
When the surge protection unit 33 is conducting to protect the LED device 40, the surge protection unit 33 withstands the surge voltage. A capability of withstanding the surge voltage of the surge protection unit 33 is directly proportional to manufacturing cost of the surge protection unit 33. When the surge protection device 30 is designed for withstanding a great surge voltage, the manufacturing cost of the surge protection unit 33 and the surge protection device 30 is also raised.